Amino Acids and Proteins

Which of the following statements about standard amino acids are correct?

P. Lysine has butylammonium side chain.

Q. Average mass of an amino acid residue is ~110 Da.

R. Universal genetic code specifies only the 22 standard amino acids.

S. Proline has a secondary amino group.

• P and Q

• Q and S

• P , Q , R and S.

• Only Q and R.

Aspartate residues are found in the active sites of many enzymes. The pKa for the beta-carboxylate of aspartate is 3.86. At physiological pH this group can function as:

• a nucleophile and a conjugate acid.

• an electrophile and a conjugate acid.

• a nucleophile and a conjugate base.

• an electrophile and a conjugate base.

Which of the following statements are true for Ramachandran Plot?

P. it can be used to describe the backbone conformation of any polypeptide residue.

Q. it can be used to describe simple secondary structures of proteins.

R. it is plotted with coordinates of psi and phi of a polypeptide chain.

S. it is used to describe side chain conformation of the amino acid residues in a polypeptide chain.

• P and Q

• P and R

• P , Q and R

• P, Q , R and S.

Which of the following statements related to the Ramachandran plot is not true?

• Almost all peptide bonds in proteins are cis.

• The restriction of psi and phi angles limits structures accessible to unfolded form of proteins.

• Three quarters of possible phi and psi values are excluded by their steric clashes.

On a ramachandran plot , the entries for hemoglobin would be clustered around:

• all four corners.

• the extended chain conformation

The peptide bond in proteins is:

• planar , but rotates to three preferred dihedral dihedral angles.

• non-polar , but rotates to three preferred dihedral angles.

• non-polar , and fixed in a trans confirmation.

• planar , and usually found in a trans configuration.